U.S. patent application number 10/656824 was filed with the patent office on 2004-03-11 for modular fuel cell.
Invention is credited to Richards, William R..
Application Number | 20040046526 10/656824 |
Document ID | / |
Family ID | 31981587 |
Filed Date | 2004-03-11 |
United States Patent
Application |
20040046526 |
Kind Code |
A1 |
Richards, William R. |
March 11, 2004 |
Modular fuel cell
Abstract
Fuel cell modules are assembled in receiver sections of an
assembly in a power generation system. Both fuel and reactant gas
feed and return lines provide leak-tight shutoff of all porting
connections to the modular fuel cell by using failsafe
(spring-loaded to the closed or "off" position) cartridge-type plug
valves in the headers. The fuel and air ports of gas distribution
manifolds on the ends of the module connect to distribution headers
using a low compression face seal that is maintained in compression
by providing tapered mating surfaces between the manifolds and
headers. Through the insertion of a module in a receiving section,
the flow control valves are actuated at the header distribution
ports, the manifold ports are aligned with the header ports with
their seals placed in compression and an electrical connection is
established between electrical plugs protruding from the module and
the base of the receiving section.
Inventors: |
Richards, William R.;
(Springfield, VA) |
Correspondence
Address: |
MATTINGLY, STANGER & MALUR, P.C.
1800 DIAGONAL ROAD
SUITE 370
ALEXANDRIA
VA
22314
US
|
Family ID: |
31981587 |
Appl. No.: |
10/656824 |
Filed: |
September 8, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
60408335 |
Sep 6, 2002 |
|
|
|
60425242 |
Nov 12, 2002 |
|
|
|
Current U.S.
Class: |
320/101 |
Current CPC
Class: |
H01M 8/2465 20130101;
H01M 8/241 20130101; H01M 8/02 20130101; H01M 8/0263 20130101; H01M
8/249 20130101; H01M 8/026 20130101; H01M 8/2483 20160201; H01M
8/248 20130101; Y02E 60/50 20130101; H01M 8/04089 20130101; H01M
8/2485 20130101 |
Class at
Publication: |
320/101 |
International
Class: |
H02J 007/00 |
Claims
I claim:
1. A modular fuel cell power generation system, comprising: a
plurality of modular fuel cells and receiving sections, each of
said modules having fuel cells in a stack between electrically
conductive end plates and at least as many receiving sections in a
base assembly having electrical bus strips and fuel gas and air
distribution headers; each of said modules further having manifolds
adjacent said end plates having ports and internal passages
connected to the ports for gas flow in supply and return passages
providing uniform supply of fuel and reactant gases to each said
fuel cell in said modules; said headers having spring loaded valves
for respective ports that supply fuel gas and air and that vent
reactive gas and air, respectively, with an actuator for each said
valve that is engaged by insertion of said module within said
receiving section when said ports of said manifolds and
corresponding ports of said headers are aligned; and said modules
having electrical connection said end plates and said base when
said module is inserted in said receiving section.
2. A modular fuel cell power generation system, according to claim
1, wherein said headers and said manifolds each have sides that
engage one another in which said ports are disposed and each of
said sides is tapered to wedge said module in between said headers
of said receiving section to establish a compressive force that
maintains a face seal between said aligned ports when said module
is inserted in said receiving section.
3. A modular fuel cell power generation system, according to claim
1, wherein said end plates have electrical plugs protruding
outwardly from said module toward said base of said receiving
section and said base has receptacles for receiving said plugs when
said module is inserted in said receiving section.
4. A modular fuel cell power generation system, according to claim
1, wherein said electrical plugs engage said sockets with
sufficient force to maintain said module in said receiving
section.
5. A modular fuel cell power generation system, according to claim
1, wherein said tapered sides of said manifolds and said headers
have a matching taper angle of approximately four degrees.
6. A modular fuel cell power generation system, according to claim
1, wherein said modules have a handle with portions of the handle
engaging the actuators of said valves when said module is inserted
in said receiving section to open gas flow through said valves.
7. A modular fuel cell power generation system according to claim
1, further including each said fuel cell including a bipolar plate
having anode and cathode gas feed sides each having grooves for
supplying gas within an individual cell; said grooves having
tapered-width micro-channel grooving that provides a significantly
improved level of fuel and reactant gas distribution uniformity
over the active area of each said cell.
8. A modular fuel cell, comprising: a plurality of fuel cells in a
stack between electrically conductive end plates, each said fuel
cell including a bipolar plate having anode and cathode gas feed
sides each having grooves for supplying gas within an individual
cell; said grooves having tapered-width micro-channel grooving that
provides a significantly improved level of fuel and reactant gas
distribution uniformity over the active area of an individual cell.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] This application claims the benefit of U.S. Provisional
Patent Application No. 60/408,335, filed Sep. 6, 2002 and U.S.
Provisional Patent Application No. 60/425,242, filed Nov. 12,
2002.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention is directed to a fuel cell module and
modular fuel cell that can be configured in an assembly having
module components capable of being easily removed and replaced. As
a preferred embodiment, the module components are capable of being
replaced and removed while the fuel cell stack is under electrical
load. This provides continuous duty/un-interruptible operation of a
series and/or parallel fuel cell stack configuration comprised of
the modules of the present invention.
[0004] 2. Description of the Related Art
[0005] PEM fuel cells are used for power generation and each of the
fuel cells has fuel and air requirements for operation. When a
number of individual fuel cells are assembled together to
constitute a module, problems can develop with the modules, and in
particular with the supply of fuel (H.sub.2) and air, as well as
with heat dissipation and power collection. In U.S. Pat. No.
6,030,718, a PEM fuel cell power system is disclosed that enables
individual fuel cell modules to be connected to racks within a
housing. The modules have a hydrogen distribution rack with a
terminal end that engages a valve on the rack that supplies
hydrogen gas to the module. The rack or housing has many slots and
each slot accepts a module. Accordingly, there are valves for
supplying hydrogen gas and a return for each slot.
BRIEF SUMMARY OF THE INVENTION
[0006] According to the present invention, a "modular",
"building-block" array of nominal 1-kW output capacity fuel cell
assemblies is provided. The modular fuel cell blocks may be
individually removed and/or replaced in an array or assembly that
enables continuous operation without interruption or significant
disruption of the power supplied by the assembly.
[0007] It is an objective to provide a lightweight (less than 10#)
and easily held fuel cell assembly of nominal 1-kW output capacity,
that may be gripped with ease and minimal force applied, to
accomplish the safe removal and/or replacement of "modular" fuel
cell assembly from a similar "modular" manifold block assembly.
[0008] It is a further object to provide the "modular" manifold
block assembly with such features that both fuel and reactant gas
feed and return lines provide positive, leak-tight shutoff and/or
isolation of all porting connections to/from the "modular" fuel
cell assembly, by the employment of Failsafe (Spring-Loaded to
Close or "Off" Position) "On-Off" control action cartridge-type
plug valve (or similar), and as effected by operator access to the
exposed front face (accessible portion) of the 1-kW fuel cell
assembly.
[0009] It is a further object to provide the "modular" manifold
block assembly with nominal 1-kW output capacity with such features
to assure that associated electrical loads may be safely and
reliably disconnected and subsequently reconnected, by the
consideration of "make-break" current levels being constrained to
values of approximately twenty (25), up to a maximum of fifty (50)
amperes per electrical plug connection.
[0010] It is a further object to provide the "modular" manifold
block assembly with features to allow porting and distribution
exhaust reactant gas flows uniformly over the exposed periphery of
the fuel cell assembly, such that convective air cooling measures
may be effected, and in conjunction with water management/cooling
processes within the fuel cell stack assembly envelope, shall limit
the maximum external surface temperatures, and thereby allow for
the safe removal of said fuel cell stack assembly by "hand" without
discomfort.
[0011] It is a further object to provide the fuel cell stack
assembly with internal fuel and reactant gas distribution features
such that both the fuel cell stack electrical output capability is
maximized, and the internal water management/cooling processes are
facilitated. This object may be accomplished, in a preferred
embodiment of the invention, by the use of slotted versus circular
internal gas distribution supply and return passages, and tapered
distribution headers, and thereby allowing for the lowest possible
velocity head losses, and the highest possible uniformity (laminar
flow) in gas flow volumes to/from all active unit area increments
of the conductive (electro-chemically active) region of the fuel
cell stack assembly.
[0012] It is a further object to provide a uniform compressive
clamping force within the conductive (electro-chemically active)
region of the fuel cell stack assembly, such that associated
contact voltage drop effects are minimized, and which results in
the achievement of optimal resistivity performance of the Gas
Diffusion Media, and which therefore yields the greatest possible
output power versus internal heat generated.
[0013] It is a further object to provide all of the above features
in a freestanding array consisting of symmetric 1-kW "modular" fuel
cell stacks capable of being installed in a series manifold of up
to twenty (20) "plug-in" elements, thereby allowing capability to
operate at 480 VDC, and via symmetric configuration, yielding
capability to provide up to 40-kW per array. The envelope of this
freestanding array of 1-kW "modular" fuel cell assemblies would be
less than 9.75" W.times.80.0" H.times.12.0" Dp. Larger-sized output
capacity arrays would be accomplished by mounting of these
freestanding arrays in parallel to realize units with 240-kW (and
larger) output capacities and providing a very small footprint.
[0014] It is yet a further object to provide such large-scale
arrays with Preventative Maintenance/Fault-Location (PM/FL)
features, to facilitate the rapid location of defective fuel cell
assemblies, such that they may be rapidly removed and replaced.
These features would preferably consist of the threshold detection
of increased temperature levels within the individual 1-kW module,
such that illumination of LEDs would indicated the operating
status. By way of example, Green would indicate operation within
room (or ambient) temperature up to 150 Deg. F., Yellow would
indicate incipient failure at greater than 150 Deg. F. for 10
minutes of longer, and Red for temperatures of greater than 180
Deg. F. Both local and remote alarms would be triggered based upon
performance/operational necessity and safety considerations.
[0015] It is a further object of the invention to include a bipolar
plate configuration, in accordance with embodiments of the present
invention, having tapered-width micro-channel grooving, which
provides a significantly improved level of fuel and reactant gas
distribution uniformity over the active area of an individual cell
itself, between a plurality of cells within a fuel cell module.
[0016] The modular fuel cell of the present invention that includes
the bipolar plates having tapered-width micro-channel grooving
facilitates the achievement of substantially reduced concentration
gradient variations over any subject unit area of the active region
of a cell (also minimizing gas flow volume/gas velocity variations
per unit area), thereby maximizes the vaporization capability of
the reactant gas stream, and which subsequently minimizes
development of "hot-spots."
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] The accompanying drawings, in conjunction with the general
description given above, and the detailed description of the
preferred embodiments given below, serve to illustrate and explain
the principles of the preferred embodiments of the best mode of the
invention presently contemplated, wherein:
[0018] FIG. 1 is a perspective view of a plurality of fuel cell
modules in an assembly for power generation configured in which the
fuel cell modules ban be easily removed and replaced;
[0019] FIG. 2 is a top view of the fuel cell modules in the
assembly shown in FIG. 1;
[0020] FIG. 3 is a side view of a plurality of fuel cell modules in
the assembly shown in FIG. 1;
[0021] FIG. 4 is a top view of a fuel cell module according to the
present invention and as shown in FIG. 1.
[0022] FIG. 5 is an end view of the fuel cell module shown in FIG.
4 that shows the front mating face of a gas distribution manifold
of the module.
[0023] FIG. 6 is a view of the back mating face of the gas
distribution manifold shown in FIG. 5.
[0024] FIG. 7 is a sectional view taken along line 7-7 in FIG.
6.
[0025] FIG. 8 is an end view of one of the collector plates used
for the fuel cell module shown in FIG. 1;
[0026] FIG. 9 is a sectional view taken along line 9-9 in FIG.
8.
[0027] FIG. 10 is an exploded perspective view of the inside
portions of a PEM fuel cell building-block module according to
another embodiment of the invention with the end plates not shown
to enable the internal components of the fuel cell to
identified.
[0028] FIG. 11 is one side view of a bipolar plate showing the
anode side gas feed groove pattern.
[0029] FIG. 12 is a sectional view of the bipolar plate shown in
FIG. 11 taken along line 12-12.
[0030] FIG. 13 is the opposite side view of the bipolar plate of
FIG. 11 showing the cathode side gas feed groove pattern.
[0031] FIG. 14 is a sectional view of the bipolar plate shown in
FIG. 11 taken along line 14-14.
DETAILED DESCRIPTION OF THE INVENTION
[0032] FIG. 1 is a perspective view of a plurality of fuel cell
module 15 in an assembly 10 for power generation configured in
which the fuel cell module 15 ban be easily removed and replaced.
In particular, a basic multi-cell module 15 with modified
manifold/collector plates is shown. In order to affect a full
"hot-swappable" capability, in accordance with an embodiment of the
present invention, the module 15 can be added and removed from an
assembly having headers and a base plate. A nominal approximate
4.degree. tapered non conductive manifold block is provided on each
end face of the module 15, such that positive sealing of a
make/break face-seal with O-Ring glands may be accomplished
simultaneously with inserting or removing of the module 15 in a
receiver section 55 to be explained in greater detail.
[0033] The base 50 of assembly 10 has bus strips 52 and 53 for the
cathode and anode connections to the fuel cell modules 15. n The
length of the base and the number of receiving sections 55
determines the overall array of fuel cell modules 15. Preferably,
the fuel cell modules 15 are disposed in a vertically extending
array by mounting the base to a vertical support structure through
mounting arrangement, not shown. Further, the output power is taken
from the array or assembly of fuel cell modules 15 by connection
with bus strips 52 and 53.
[0034] Referring to FIG. 2, plunger-actuated "failsafe" cartridge
check valves 60 (H.sub.2 vent), 61 (air out), 62 (air in) and 63
(H.sub.2 in) are provided for the make/break isolation of both fuel
(H.sub.2) and reactant gas (air) connections. This ensures a low
(e.g., <10#) insertion/removal force associated with the
insertion/removal of the module 15 with respect to the assembly.
Any practical number of fuel cell modules 15 can be assembled
together in the manner shown in the figure. Parallel stacking of
10-20 1.0-kW Modules 15 is preferred depending on flow provided
through the primary gas distribution headers within the receiver
sections 55. The exhaust air flow of each respective module 15
provides external cooling by redirection of this air flow over the
external (exposed) bipolar plates 90 in the module 15 or fuel cell
stack. Further, the capability is provided to achieve a positive
orientation and alignment of a fuel cell module 15 within its
respective receiver via the use of keyed relief features and
placement of electrical plug receptacles 79 to generate an
interference if the orientation is incorrect. According to the
invention, a plurality of modules 15 are connected electrically in
series to generate a DC voltage output capability that depends on
the number of fuel cells that are in the assembly.
[0035] FIG. 2 shows an example of as complete 1-kW fuel cell module
15 coupled with a receiver section 55, in accordance with an
embodiment of the present invention. The fuel cell module 15
consists of a stack of fuel cells 25, shown in greater detail in
FIG. 10. The stack is bolted together with tie rods 5 having end
nuts. At the front of each module 15 is a handle 16 having
projected end pieces 17 (shown schematically in FIG. 2). End pieces
17 engage the actuators 65-68 of the valves 60-63 when the module
15 is inserted in the receiver section 55. Since the valves are
normally spring biased in the closed position, the gas and air is
supplied through the respective ports 41-44 in headers 40, which
are connected to the appropriate gas supply through common piping
45-48 or ports (shown plugged).
[0036] FIG. 3 is a side view of three fuel cell modules 15a, 15b
and 15c with two of the modules 15a, 15b seated within the
respective receiver sections 55a, 55b and with a third module 15c
aligned with the receiver section 55c and not fully seated. The
electrical plugs 80 that are shown for the modules 15 are fixed on
the collector plate 98 as shown in FIGS. 8 and 9. The plugs may be
banana plugs or an equivalent plug that enables electrical
connection with the base and that also preferably secures the
module 15 in place. Alternatively, the plug may be a spade type
that is received within the receiver section in combination with a
locking device that holds the module 15 in place. Further, a
combination of the banana plugs and a separate locking device can
be used to secure the modules 15 in place.
[0037] FIG. 4 is a top view of the fuel cell module 15 according to
the present invention. Tie rods 5 are shown holding the stack of
fuel cells 25 together between end plates 98. The taper of gas
distribution manifolds 82 is shown, which is preferably 4.degree.,
or within a range of taper that permits the engagement of the seals
for the gas and air connections without excessive force, while
maintaining a force against the seals that ensures gas tight
connection.
[0038] FIG. 5 is an end view of the fuel cell module 15 shown in
FIG. 4 that shows the front mating face for a gas distribution
manifold 82 of the module 15. The ports 83, 84 are for H.sub.2 gas
in and vent, respectively, and ports 85, 86 are for air in and air
out. FIG. 6 is a view of the back mating face of the gas
distribution manifold shown in FIG. 5 and FIG. 7 is a sectional
view taken along line 7-7 in FIG. 6. The gas distribution channels
87 (H.sub.2), 88(air) within the manifold are slot shaped (in cross
section). The slots are rectangular in overall shape with rounded
end portions that are approximately semicircular. Preferably, the
rectangular dimensions are 4 to 1.about.10 to 1 in length to width
dimensions with semicircular end portions that have a diameter
equal to the width dimension. The ports fan out as shown at 89 to
connect with the slot shaped gas distribution passages as shown in
FIG. 6.
[0039] As shown in FIGS. 4 and 7, the manifolds have a tapered side
81 with ports 83-86 having respective O ring seals for face to face
connection with ports 41-44 in the headers. The respective ports of
the headers and the manifolds are aligned with one another as shown
in FIG. 3 when a module 15 is seated within a receiving section 55.
Further, the insertion of the module 15 causes extension pieces 17
projecting outwardly from the handle to engage plunger actuators
65-68 of the spring loaded cartridge vales 60-63 as shown in FIGS.
1 and 2 (shown schematically in FIG. 2 just prior to engagement).
When the handle extension pieces 17 depress the actuators 65-68,
the valves 60-63 are opened from their normally closed position and
the fuel gas and reactant gas (air) is supplied through the aligned
ports to the manifolds of the module 15. Likewise, when the module
15 is removed from the receiving section 55, the spring bias of the
valves forces the valves to the closed position to prevent escape
of gas to the atmosphere. Further, insertion of the module 15
establishes electrical connection of the module 15 with the base 50
through electrical plugs 80 and mating sockets 79 provided in the
base. Although both sides of the module 15 are shown to have
tapered surfaces to achieve compression of the seals for engaging
similarly tapered surfaces of the headers, only one side of the
manifolds/headers could be tapered, however the preferred
embodiment shows that both sides are tapered for ensuring equal
compressive force distribution across the seals of the aligned
ports to prevent leaks.
[0040] FIG. 8 is an end view of a collector end plate 98 for the
fuel cell module 15 and FIG. 9 is a sectional view taken along line
9-9 in FIG. 8. There are two collector end plates 98, one at each
end of the module 15 that are made of an electrically conductive
material. All of the voltage generated by the module 15 is
collected by the end collector plates 98, one cathode and one
anode. Further, on only the internal face of each collector plate
(facing the stack) grooving for gas distribution is provided. One
plate has grooving for distribution of H.sub.2 and the other plate
has grooving for air distribution. Each plate has gas distribution
passages 93 (H.sub.2) and 94(air) and tie rod through holes 96, as
shown in the figures. The through holes 96 are provided for the
bolts or tie rods 5 that hold the module 15 together. The internal
tie rods 5 are electrically isolated from each of the individual
fuel cells and from the manifolds and collector end plates, which
serve to establish a uniform compressive stress of, for example,
<.+-.1% over the active area of the cells within the module 15.
Flanged bushings 97 in the collector plate through holes 96 also
ensure electrical isolation of the tie rods and nuts for securing
the plates together. The connection between the manifolds and the
end plates are insulated also, as shown in FIG. 2.
[0041] Electrical plugs 80, such as banana plugs are threaded into
one edge 91 of the plate as shown in FIG. 8. All of the voltage
generated by the module 15 is collected by the end collector
plates, one cathode and one anode and the plugs respectively
transfer the current to the conductor strips 52, 53 in the base 50
of the assembly, as shown in FIG. 1. Accordingly, the base is also
of a material that is an electrical insulator in order to
accommodate the conductive bus strips.
[0042] FIG. 10 is a perspective view (exploded view) of the inside
portions of a PEM fuel cell to be described for the purpose of
illustrating an embodiment of fuel cell 25, which makes up the fuel
cell module 15. This illustration depicts both a tie rod through
hole pattern, located at the corners of the individual cell
component elements, and a fuel and reactant gas distribution hole
pattern located at mid-points between the tie rod through holes.
Whereas circular gas distribution passages are shown in this
embodiment, preferably the gas distribution passages 25a, 26a, 27a
and 28a are slot shaped in cross section with rectangular
dimensions that are 4 to 1.about.10 to 1 in length to width
dimensions and with semicircular end portions that have a diameter
equal to the width dimension.
[0043] referring to FIG. 10, a module 15 typically has 1 of the 40
cells utilized to generate a nominal 5-kW of output power of 25 VDC
at 200 amperes. A single cell's overall thickness regardless of the
size of the active area chosen for the design is approximately
0.080 inches, with an active area (darkened center portion of item
number 23) of approximately 250 cm.sup.2. An individual cell
consists of an upper anode fuel gas distribution pattern as
depicted in phantom dotted lines on the bipolar plate item 20a and
a lower cathode reactant gas distribution pattern on the lower
bipolar plate 20b positioned at right angles to that of the fuel
gas distribution pattern. Sandwiched between these two plates are a
membrane electrode assembly (MEA) 23, which is itself sandwiched
between a set of rigid non-conductive gaskets 21 with associated
gas diffusion media (GDM) 22. The benefit of slotted gas
distribution channels is described in U.S. application Ser. No.
10/393,919, which is hereby incorporated herein by reference.
[0044] A preferred embodiment of a bipolar plate 90 is shown in
FIGS. 11-14. The bipolar plates are used in conjunction with
precision thickness, rigid, non-metallic gaskets to achieve the
compressive deformation of the Gas Diffusion Media, such that both
through-plane and in-plane gas permeability characteristics are
precisely controlled between a plurality of cells within a fuel
cell module 15 or stack, and within an individual cell itself.
FIGS. 11 and 12 show the anode side of the bipolar plate 90 whereas
FIGS. 13 and 14 show the cathode side of the plate.
[0045] The bipolar plate shown in FIGS. 11-14 has closely-spaced
(interdigitated) tapered-width grooves 100 (H.sub.2), 106 (air)
possessing "large" groove depth in relation to the characteristic
width, from 0.5.times. to 1.times. that of the minimum width
dimension. The tapered-width groove facilitates the establishment
of a highly uniform gas flow per unit area, by assuring, via the
use of a "mirror-image" adjacent tapered grooving, that the inlet
pressure drop is identically the same as the outlet pressure drop,
regardless of the unit area being considered within the total
active area of the cell. In one embodiment, the gas flow path
lengths, and associated restrictive losses, are identically the
same or very similar for any successive unit area's inlet pressure
drop plus that of the outlet pressure drop.
[0046] The bipolar plate 90 shown in FIGS. 11-14 has gas
distribution header slots 101 (H.sub.2), 102 (air) that are used in
the present invention, rather than holes, which are typically used
in the prior art. The slots, are adjacent associated "gates" 103
(H.sub.2), 104 (air) consisting of a plurality of closely spaced
standoffs adjacent to the inlet and/or outlet of the slot. This
facilitates the generation of controlled inlet and/or outlet
pressure drop, which is significantly greater than the pressure
drop that exists within the distribution header itself. The
resultant effect is analogous to "desensitizing" an individual cell
within a fuel cell module 15 from pressure gradients in the inlet
and outlet distribution header pressure levels. This resultant
effect is further described as creating a condition wherein each
cell is connected to a gas distribution header with an "apparently"
constant inlet pressure and/or back pressure, regardless of the
cell's location within the module 15, such that each and every cell
within the fuel cell module 15 "sees" an identically same or very
similar differential pressure existing across the cell. The net
effect is to establish an operational state whereby each cell is
able to consume identically or nearly the same amount of either
fuel or reactant gas (presupposing that each cell possesses
approximately the same gas flow volume per pressure drop resistance
characteristic).
[0047] The bipolar plate design approach shown in FIGS. 11-14 is
further depicted by the use of a tapered-width distribution header,
which is utilized to feed the plurality of closely-spaced
(interdigitated) tapered-width grooves. The net effect of
employment of this header configuration is to realize a condition
of essentially constant inlet pressure and/or outlet pressure, such
that a high degree of gas flow volume uniformity may be achieved
over the entire active area of the cell itself, by, for example,
the virtual elimination of any pressure gradient that might have
existed from the center of the header to the outermost portion of
the header if a non-tapered width header configuration (or similar)
were employed.
[0048] The bipolar plate design approach shown in FIGS. 11-14 is
achieved by fabricating the pattern of grooves 100, 106 and gates
103, 104 with control of the associated groove depths via a process
of photo-engraving or similar conventional method. It is possible
to tailor the bipolar plate design configuration to the Gas
Diffusion Media (GDM) permeability characteristic by observing that
the grooving depth dominates the gas flow versus pressure drop
characteristic of the individual tapered-width grooves. That is,
the flow volume per unit time is proportional to the product of the
values of the third power of the depth dimension and the pressure
drop. It may therefore be seen that, for GDM materials possessing a
relatively low permeability, the present invention allows reduction
in the required grooving depths. Conversely, if the GDM material
has a high permeability, then the grooving depths can be increased
in order to maintain an essentially constant delivery or return
pressures within the respective "headers" (e.g., the tapered-width
grooves).
[0049] The resultant bipolar plate design, in accordance with
embodiments of the present invention, facilitates the use of the
GDM's through-plane and in-plane permeability factors to yield a
highly consistent gas flow volume per unit area resistance effect,
which thereby allows it to generate a highly uniform (e.g.,
identical) pressure drop for any specific unit area region, as
supplied by its associated inlet and outlet tapered-width grooves.
A highly uniform flow volume per unit area is consequently
achieved. This consistency in the GDM material resistance (gas flow
volume per unit area versus the pressure drop) is primarily
realized by use of the precision rigid gaskets (see, e.g., FIG. 10)
within each cell configuration to control the overall thickness
variation of the compressed GDM to a very high tolerance, typically
less than .+-.0.00025", for example, over the entire active area of
the cell configuration. Additionally, this "as-realized" compressed
thickness of the GDM also provides a controlled, highly consistent,
minimum value for the resultant resistivity (Ohms-cm.sup.2). This
resistivity may readily be determined via Micro-Ohm Meter
measurements of a test sample GDM's electrical resistance
(Ohms.times.Sample Size, cm.sup.2) versus its imposed compressive
stress. One net effect of using precision gaskets is to facilitate
the achievement of a highly consistent cell resistivity per unit
area (by a precise control over the GDM deformation, and subsequent
level of compressive stress) and to simultaneously realize an
optimally low value for the resultant GDM resistivity. This yields
the lowest possible value for cell resistivity, as measured by the
effects of the individual cell's MEA, its electrodes, GDMs, and
associated Bipolar Plates.
[0050] In conjunction with the previously identified capability to
achieve a highly uniform gas flow volume per unit area, it can
subsequently be demonstrated that a highly consistent
electro-chemical process may be established per unit area, with
electro-chemical plus electro-osmotic processes, including
stoichiometric fuel and reactant gas states and operating
conditions, that is virtually identical from anyone unit area to
another unit area within the active region of a cell. The resultant
configuration therefore yields greatly reduced susceptibility to
"hot-spot" generation, and allows the establishment of a highly
uniform current density over the entire active area of the cell.
Furthermore, it may also be demonstrated that this same consistency
exists between any successive cell within the fuel cell stack, and
that the resultant variations in the measured cell to cell output
voltages may be kept to very small values (e.g., <.+-.0.010 V
DC).
[0051] While preferred embodiments have been set forth with
specific details, further embodiments, modifications and variations
are contemplated according to the broader aspects of the present
invention, all as determined by the spirit and scope of the
following claims.
* * * * *